Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In some RFID tags the power management section includes an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
The amount of energy that a tag can extract from an incident electromagnetic field varies with the orientation of the tag's antenna relative to the electromagnetic field. In certain orientations, a passive tag may not be able to extract sufficient energy to power itself. As a result, a reader's ability to read tags within its field of view may be reduced depending on the orientation of the tag antenna relative to the reader antenna. Some tags include two antennas with different orientations to increase the tag's ability to extract power from the incident field; these tags are often called dual-antenna tags. In conventional dual-antenna tags the antenna ports within an integrated circuit (IC) of the tag share a reference potential. Unfortunately, as the tag dimensions become small relative to the wavelength of the electromagnetic radiation, the tag antennas couple electromagnetically and, as a consequence of the shared reference potential, act electrically like a single antenna, thereby negating the orientation-insensitivity benefits of the dual antennas.